Metal members

ABSTRACT

The invention provides metal members having liquid-repellent and corrosion-resistant surfaces, without the need for SAM surface treatment. A metal member of the disclosure has a porous surface, having the porous surface directly covered by a hydrocarbon-based oil comprising zinc dialkyldithiophosphate (ZnDTP). The porous surface may be an oxidized surface, and especially an anodized surface. The metal member may be a member of Al, Ti, Fe or Mg, or an alloy of any of these metals, or stainless steel. The concentration of the zinc dialkyldithiophosphate (ZnDTP) may be 0.1 mass % to 30.0 mass % with respect to the hydrocarbon-based oil.

FIELD

The present disclosure relates to metal members.

BACKGROUND

There is existing industrial demand for metal members with liquid-repellent surfaces.

One type of known liquid-repellent surface is a slippery liquid surface referred to as Slippery Liquid-Infused Porous Surface (SLIPS), which is a liquid-repellent surface comprising a rough surface with low surface free energy and a fine irregular structure, and a lubricant oil film coated over it.

A rough surface with low surface free energy improves the wetness of lubricant oils and prevents firm sticking of liquids to the solid surface, so that droplets that have stuck onto lubricant oil films have high mobility and easily slide down even gentle slopes.

Because SLIPS have such features, they are not only useful as liquid-repellent, antifouling and corrosion-resistant surfaces, but are also expected to allow applications and deployments that have been difficult to achieve with conventional super-water-repellent or super-oil-repellent surfaces, including applications for ice-resistant surfaces that prevent sticking of ice, biological contaminant-resistant surfaces that prevent sticking of biological liquids such as blood, and water harvest technologies that utilize the high droplet-aggregating properties of lubricant oil films.

Typical examples of substances with low surface free energy include fluorine-based resins such as polytetrafluoroethylene (PTFE, critical interfacial tension γC=18 mNm⁻¹), but their uses have so far been limited.

Liquid repellency for a variety of metal oxide surfaces has also been in great demand in the industry. The surface free energy values for metal oxides are much higher than for other substances, but it is known that surface free energy can be vastly reduced by modifying the surface with a self-assembled monolayer (SAM) comprising long-chain perfluoroalkyl groups with —CF₃ (γC=6 mNm⁻¹) at the ends, or long-chain alkyl groups with —CH₃ (γC=24 mNm⁻¹) at the ends. Phosphonic acid derivatives, which have come to be widely used in recent years, are highly stable compounds that form denser and more stable SAMs than conventionally known silane coupling agents.

For steel materials of automobile engine parts, it is known from PTL 1 that their impact resistance, abrasion resistance and corrosion resistance can be increased by surface treatment of the steel materials by a method of plasma nitriding followed by injection of nitrogen ions to a nitrogen concentration of 30 atomic percent or greater.

CITATION LIST Patent Literature

-   [PTL 1] Japanese Unexamined Patent Publication No. 2003-073800

SUMMARY Technical Problem

Chemical substances used to form SAMs are expensive, with coating with SAMs by liquid phase methods requiring strict solution management, and costly equipment being necessary for gas phase methods.

Thus, the ability to produce metal members with liquid-repellent surfaces without requiring SAM surface treatment would be expected to simplify the processing steps and vastly reduce costs for large-scale industrial purposes.

However, SAM surface treatment for reduction of surface free energy on metal oxide surfaces has been considered to be essential for causing stuck liquids to slide off.

The present inventors have in fact found that even when using lubricant oils composed of fluorine-based polymers, or silicone oils, that are well known as SLIPS lubricant oils, sticking of water cannot be prevented unless SAM surface treatment is also carried out.

It is an object of the present disclosure to provide metal members having water-repellent and corrosion-resistant surfaces, without the need for SAM surface treatment.

Solution to Problem

The present inventors have found that this object can be achieved by the following means.

<Aspect 1>

A metal member having a porous surface, wherein the porous surface is directly covered by a hydrocarbon-based oil comprising zinc dialkyldithiophosphate (ZnDTP).

<Aspect 2>

The metal member according to aspect 1, wherein the porous surface is an oxidized surface.

<Aspect 3>

The metal member according to aspect 2, wherein the porous surface is an anodized surface.

<Aspect 4>

The metal member according to any one of aspects 1 to 3, wherein the metal member is a member of Al, Ti, Fe or Mg, or an alloy of these metals, or stainless steel.

<Aspect 5>

The metal member according to any one of aspects 1 to 4, wherein the concentration of the zinc dialkyldithiophosphate (ZnDTP) is 0.1 mass % to 30.0 mass % with respect to the hydrocarbon-based oil.

<Aspect 6>

The metal member according to any one of aspects 1 to 5, wherein the hydrocarbon-based oil is an engine oil.

<Aspect 7>

The metal member according to any one of aspects 1 to 6, which is a member for an automobile.

<Aspect 8>

The metal member according to aspect 7, which is to be used for a part to which an engine oil is to be supplied.

<Aspect 9>

The metal member according to aspect 7 or 8, which is a member for an intercooler.

Advantageous Effects of Invention

According to the present disclosure it is possible to provide metal members having water-repellent and corrosion-resistant surfaces, without the need for SAM surface treatment.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a scanning electron microscope (SEM) image of the surface of an aluminum sheet with a porous surface.

FIG. 1B is a scanning electron microscope (SEM) image of the surface of an aluminum sheet with a hierarchically structured porous surface.

FIG. 1C is a diagram showing contact angles of water droplets and automobile engine oil droplets on the surfaces of each of the aluminum sheets of Reference Examples 1 to 4.

FIG. 2A is a graph showing the relationship between aluminum sheet rotational speed and automobile engine oil residue amount (rotation time: 60 seconds), for each of the aluminum sheets of Examples 1 and 2 and Comparative Example 5.

FIG. 2B is a graph showing the relationship between aluminum sheet rotational speed and 10 μL water droplet falling angle (rotation time: 60 seconds), for each of the aluminum sheets of Examples 1 and 2 and Comparative Example 5.

FIG. 2C is a graph showing the relationship between automobile engine oil residue amount and 10 μL water droplet falling angle (rotation times: 10, 60 and 300 seconds), for each of the aluminum sheets of Examples 1 and 2 and Comparative Example 5.

FIG. 3 is a graph showing the relationship between liquid mixture immersion time and 10 μL water droplet falling angle, for the aluminum sheets of Examples 1 and 2 and Comparative Examples 5 and 7.

FIG. 4 is a graph showing weight change after a corrosion resistance test for the aluminum sheets of Examples 1 and 2 and Comparative Examples 1 to 6.

FIG. 5A is a scanning electron microscope (SEM) image of the surface of an anodized aluminum sheet without hierarchical structuring, before a corrosion resistance test.

FIG. 5B is a scanning electron microscope (SEM) image of the surface of the aluminum sheet of Comparative Example 5 after the corrosion resistance test.

FIG. 5C is a scanning electron microscope (SEM) image of the surface of the aluminum sheet of Comparative Example 3 after the corrosion resistance test.

FIG. 5D is a scanning electron microscope (SEM) image of the surface of the aluminum sheet of Example 1 after the corrosion resistance test.

FIG. 5E is a scanning electron microscope (SEM) image of the surface of the aluminum sheet of Comparative Example 1 after the corrosion resistance test.

FIG. 6A is a scanning electron microscope (SEM) image of the surface of a hierarchically structured and anodized aluminum sheet, before a corrosion resistance test.

FIG. 6B is a scanning electron microscope (SEM) image of the surface of the aluminum sheet of Comparative Example 6 after the corrosion resistance test.

FIG. 6C is a scanning electron microscope (SEM) image of the surface of the aluminum sheet of Comparative Example 4 after the corrosion resistance test.

FIG. 6D is a scanning electron microscope (SEM) image of the surface of the aluminum sheet of Example 2 after the corrosion resistance test.

FIG. 6E is a scanning electron microscope (SEM) image of the surface of the aluminum sheet of Comparative Example 2 after the corrosion resistance test.

FIG. 7 is a diagram illustrating the method of fabricating the aluminum sheet of Reference Example 5.

FIG. 8A is a diagram showing the state of the surface of the aluminum sheet of Reference Example 5 and the contact angle upon placement of a 10 μL water droplet.

FIG. 8B is a diagram showing the state of the surface of the aluminum sheet of Reference Example 6 and the contact angle upon placement of a 10 μL water droplet.

FIG. 9A is a graph showing X-ray photoelectron spectroscopy (XPS) analysis results for the surface of the aluminum sheet of Reference Example 5, after automobile engine oil immersion and cleaning.

FIG. 9B is a graph showing X-ray photoelectron spectroscopy (XPS) analysis results for the surface of the aluminum sheet of Reference Example 5, after automobile engine oil immersion and cleaning.

FIG. 9C is a graph showing X-ray photoelectron spectroscopy (XPS) analysis results for the surface of the aluminum sheet of Reference Example 5, after automobile engine oil immersion and cleaning.

FIG. 10 is a graph showing electrochemical measurement results for the aluminum sheets of Examples 3 to 5 and Comparative Example 8.

FIG. 11 is a graph showing electrochemical measurement results for the aluminum sheets of Example 3 and Comparative Examples 8 to 10.

DESCRIPTION OF EMBODIMENTS

Embodiments of the disclosure will now be described in detail. The disclosure is not limited to the embodiments described below, however, and various modifications may be implemented which do not depart from the gist thereof.

A metal member of the disclosure has a porous surface, with the porous surface directly covered by a hydrocarbon-based oil comprising zinc dialkyldithiophosphate (ZnDTP).

<Metal Member>

According to the disclosure, the metal member has a porous surface.

For this purpose, a porous surface is a porous layer formed on the surface of a metal member, and for example, it may be an oxidized surface with an oxide of the same type of metal as the metal member. More specifically, the porous surface may be an anodized surface.

The anodized surface can be obtained by anodization of the metal member, and when the metal member is an Al member, for example, it can be obtained by alumite treatment, and more specifically by surface treatment that produces an oxide layer (Al₂O₃) by electrolysis using Al as the anode (positive electrode). Alumite treatment can be carried out according to JIS H8601 or JIS H8603, for example, although another method may also be used.

The thickness and pore sizes of the porous surface are not particularly restricted.

The thickness of the porous surface can be appropriately adjusted according to the purpose of use of the metal member.

The form of the porous surface is not restricted so long as it is a slippery liquid surface (SLIPS). The pore sizes (diameters) of each of the pores of the porous surface are preferably 1 nm to 5 μm. The porous surface may have a microscale irregular structure, a nanoscale irregular structure or a mixture of such structures, or a hierarchical structure comprising them.

For example, when the porous surface has a microscale irregular structure, the pore sizes (diameters) of each of the pores of the porous surface may be 0.1 μm to 5 μm, but they are preferably 0.1 μm to 0.5 μm.

When the porous surface has a nanoscale irregular structure, the pore sizes (diameters) of each of the pores of the porous surface may be 1 nm to 100 nm, but they are preferably 30 nm to 100 nm.

The material of the metal member may be any metal that is able to form a porous surface, and for example, the metal member may be a member composed of Al, Ti, Fe or Mg, or an alloy of any of those metals, or stainless steel.

The metal member may be a member for any desired purpose of use, such as a vehicle member, and more specifically an automobile member.

When the metal member is an automobile member, it is preferably a part to which engine oil is to be supplied. When the metal member of the disclosure is applied to such a part, it is possible to provide a constant supply of engine oil (as the hydrocarbon-based oil) to the metal member even when the engine oil dissociates from the surface of the metal member.

When the metal member is an automobile member, the metal member may be an intercooler member. An intercooler is a unit with inflow of high-temperature air, and since exhaust gas also flows in due to the construction of the automobile, it is one unit of the automobile whose members are especially prone to corrosion. One such member, for example, is the heat exchanger. The corrosion resistance of the intercooler can therefore be improved by applying a metal member according to this disclosure, for such members.

<Hydrocarbon-Based Oil>

The hydrocarbon-based oil includes at least zinc dialkyldithiophosphate (ZnDTP). The hydrocarbon-based oil may be a paraffinic oil or a polyalphaolefin. The hydrocarbon-based oil may be a lubricant oil, specifically an engine oil, and more specifically an engine oil for an automobile.

When included in a hydrocarbon-based oil, zinc dialkyldithiophosphate (ZnDTP) imparts water-repellency to the surface of the metal member. More specifically, it is thought that when a hydrocarbon-based oil containing zinc dialkyldithiophosphate (ZnDTP) is coated onto a metal member, it is chemically adsorbed onto the surface of the metal member, forming a liquid-repellent coated film.

The concentration of the zinc dialkyldithiophosphate (ZnDTP) may be 0.1 mass % to 30.0 mass % with respect to the hydrocarbon-based oil. The concentration of the zinc dialkyldithiophosphate (ZnDTP) may be 0.1 mass % or greater, 1.0 mass % or greater, 2.0 mass % or greater or 5.0 mass % or greater, and 30.0 mass % or lower, 20.0 mass % or lower, 10.0 mass % or lower or 5.0 mass % or lower.

EXAMPLES Reference Examples 1 to 4

Metal members for Reference Examples 1 to 4 were fabricated and evaluated for liquid repellency, in the following manner.

Reference Example 1

A 99.5% aluminum (Al) sheet was cut out to a size of 20 mm×50 mm and subjected to acetone degreasing by ultrasonic cleaning for 10 minutes. It was then immersed for 120 seconds in a 1.0 M NaOH aqueous solution (60° C.) to remove the oxide layer, and then immersed for 180 seconds in a 1.0 M HNO₃ aqueous solution (60° C.) to remove the smut produced in the previous step.

Next, anodization was carried out in a 0.3 M H₂SO₄ aqueous solution (15° C.) for 180 seconds with a voltage of 25 V between the polar plates, forming an anodized layer having pores on the nanometer scale. The procedure was carried out in a two-electrode system with this sheet as the working electrode and a separate Al sheet as the counter electrode.

It was then immersed for 15 minutes in a 5 wt % H₃PO₄ aqueous solution (30° C.) for expansion of the pore sizes.

Finally, oxygen plasma treatment was carried out for 4 minutes for cleaning of the surface, to obtain an aluminum sheet having a porous anodized surface.

Reference Example 2

An aluminum sheet with a hierarchical structure was obtained in the same manner as Reference Example 1, except that chemical etching was carried out before anodization, forming etch pits on the micrometer scale.

Reference Example 3

The aluminum sheet of Reference Example 1 was subjected to oxygen plasma treatment, and then immersed for 2 days in an ethanol solution containing 1 mM CF₃(CF₂)₇PO(OH)₂ (perfluorooctylphosphonic acid, FOPA), and finally heat treated for 1 hour in an air atmosphere (100° C.), to form a self-assembled monolayer (SAM) on the surface of the aluminum sheet of Reference Example 1.

Reference Example 4

A self-assembled monolayer (SAM) was formed on the surface of the aluminum sheet of Reference Example 2 having a hierarchical structure, in the same manner as Reference Example 3, except for using the aluminum sheet of Reference Example 2 instead of the aluminum sheet of Reference Example 1.

<Measurement of Liquid Repellency> (Measuring Method)

Each of the aluminum sheets of Reference Examples 1 to 4 were measured for water droplet or automobile engine oil droplet contact angle.

The aluminum sheets of Reference Examples 1 and 3 which did not have a hierarchical structure were measured for static contact angle, while the aluminum sheets of Reference Examples 2 and 4 which had a hierarchical structure were measured for forward contact angle (dynamic contact angle) and contact angle hysteresis.

(Results)

The constructions and evaluation results for Reference Examples 1 to 4 are shown in Table 1 and FIGS. 1A to 1C. In the table, the values in parentheses in the contact angle column indicate the contact angle hysteresis.

TABLE 1 Results Structure Contact angle evaluation (°) Hierar- Automobile Example SAM chical Water engine oil Reference Example 1 No No  4.6 ± 1.2 32.9 ± 0.9 Reference Example 2 No Yes ~0 7.2 ± 0.3 (4.4 ± 1.3) Reference Example 3 Yes No 129.1 ± 0.8 85.4 ± 1.9 Reference Example 4 Yes Yes 161.7 ± 1.0 158.2 ± 1.5 (23.6 ± 5.6)

FIG. 1A is a scanning electron microscope (SEM) image of the aluminum sheet of Reference Example 1. As shown in FIG. 1A, a porous Al₂O₃ layer had formed on the surface of the aluminum sheet of Reference Example 1 by anodization. In addition, as shown in FIG. 1B, micrometer-scale pits produced by etching and a hierarchical structure due to the porous Al₂O₃ layer had been formed on the surface of the aluminum sheet of Reference Example 2.

As shown in Table 1 and FIG. 1C, the aluminum sheets of Reference Examples 1 and 2 which had not formed a self-assembled monolayer (SAM) were super hydrophilic, having water droplet contact angles of 4.6±1.2 and approximately 0, respectively. In contrast, the aluminum sheets of Reference Examples 3 and 4 which had formed self-assembled monolayers (SAM) had high water-repellency, with water droplet contact angles of 129.1±0.8 and 161.7±1.0, respectively.

With the aluminum sheet of Reference Example 4 which had a hierarchical structure and a self-assembled monolayer (SAM) formed on the surface, the contact angle was a large value of 158.2±1.5, with high liquid repellency for automobile engine oil droplets, whereas in Reference Examples 1 to 3 the contact angle was 90°, which was low liquid repellency.

This indicates that it is possible to form an automobile engine oil as an oil film at least on the aluminum sheets of Reference Examples 1 to 3.

Examples 1 and 2 and Comparative Examples 5 to 7

Aluminum sheets for Examples 1 and 2 and Comparative Examples 5 to 7 were fabricated and their performance evaluated, in the following manner.

Examples 1 and 2 and Comparative Examples 5 and 6

A 100 μL portion of the automobile engine oil was measured out, coated onto the surfaces of each of the aluminum sheets of Reference Examples 1 to 4 and allowed to stand for 10 minutes or longer, to obtain aluminum sheets for Examples 1 and 2 and for Comparative Examples 5 and 6.

Incidentally, since the automobile engine oil did not sufficiently spread wet on the surface of the aluminum sheet of Reference Example 4 which had a hierarchical structure and a SAM coating, as demonstrated in the evaluation of liquid repellency of the automobile engine oil for Reference Example 4, the automobile engine oil was therefore coated after immersion beforehand for 48 hours in the engine oil to reduce the oil repellency.

Comparative Example 7

An aluminum sheet for Comparative Example 7 was obtained in the same manner as Example 1, except for coating the automobile engine oil on an aluminum sheet lacking hierarchical structuring by etching, porosity by anodization and SAM coating formation.

<Evaluation of SLIPS Stability in Environment with Shear Force Applied to Automobile Engine Oil>

(Evaluation Method)

The aluminum sheets of Examples 1 and 2 and Comparative Example 5 were evaluated for automobile engine oil retentivity and liquid sliding properties in an environment with shear force applied to the automobile engine oil. This served to simulate reduction of SLIPS in an environment causing loss of lubricant oil due to gravity or wind.

For the evaluation method, first, each of the aluminum sheets of Examples 1 and 2 and Comparative Example 5 were rotated using a spin coater under conditions with a rotational speed of 500 rpm to 7000 rpm and a rotation time of 10 seconds to 300 seconds. The weight of the remaining automobile engine oil and the 10 μL water droplet falling angle were then measured.

(Results)

The measurement results are shown in FIGS. 2A to C. FIG. 2A is a graph showing the relationship between aluminum sheet rotational speed and automobile engine oil residue amount (rotation time: 60 seconds), for each of the aluminum sheets of Examples 1 and 2 and Comparative Example 5, FIG. 2B is a graph showing the relationship between aluminum sheet rotational speed and 10 μL water droplet falling angle (rotation time: 60 seconds), for each of the aluminum sheets of Examples 1 and 2 and Comparative Example 5, and FIG. 2C is a graph showing the relationship between automobile engine oil residue amount and 10 μL water droplet falling angle (rotation times: 10, 60 and 300 seconds), for each of the aluminum sheets of Examples 1 and 2 and Comparative Example 5.

As shown in FIG. 2A, for all of the aluminum sheets, raising the rotational speed to increase the centrifugal force on the automobile engine oil caused a corresponding reduction in the automobile engine oil residue amount.

The residue amount was especially large with the aluminum sheet of Example 2 which had a hierarchical structure. This is attributed to the aluminum sheet with a hierarchical structure having micrometer-scale irregularities in which the automobile engine oil was retained.

Comparison between the aluminum sheets of Example 1 and Comparative Example 5 which did not have a hierarchical structure showed no significant difference in automobile engine oil residue amount.

In regard to the water droplet falling angle, as shown in FIG. 2B, it was confirmed that coating of an automobile engine oil on the porous surface caused adhered water droplets to slide off at a given falling angle, even with the aluminum sheet of Example 1 which did not have a hierarchical structure and also lacked a SAM coating. Since the surface of the aluminum sheet of Example 1 itself is hydrophilic as shown in FIG. 1C, presumably it cannot function by itself as a SLIPS.

Upon comparing the aluminum sheets of Examples 1 and 2 and Comparative Example 5, the water droplet falling angle increased, i.e. the water-repellency decreased, as the rotational speed increased, with greater tendency to decrease being exhibited with the aluminum sheet of Example 2 which had a hierarchical structure but lacked a SAM coating, while conversely, the smallest water droplet falling angle was maintained, i.e. greater water-repellency was maintained, with the aluminum sheet of Example 1 which did not have a hierarchical structure and also lacked a SAM coating.

The reason that the water droplet falling angle particularly tended to increase with the aluminum sheet of Example 2 is thought to be because the originally smooth lubricant oil film reflected greater roughness of the surface as the lubricant oil was lost, leading to roughening of the lubricant oil film and increased contact area with the water droplets.

Based on the relationship between the automobile engine oil residue amount and 10 μL water droplet falling angle as well, the smallest water droplet falling angle (satisfactory water-repellency) even with the same lubricant oil residue amount, was exhibited by the aluminum sheet of Example 1 which did not have a hierarchical structure and lacked a SAM coating.

Thus, as shown in FIG. 2C, the surfaces having automobile engine oil directly coated onto aluminum sheets that lacked a SAM coating, as in Examples 1 and 2, not only allowed water droplets to easily slide down, similar to a surface having a SAM coating formed on an aluminum sheet as in Comparative Example 5, but also maintained the highest stability even in an environment with shear force applied to the lubricant oil.

<Evaluation of SLIPS Stability in Water> (Evaluation Method)

Acetic acid and a 10 g/L NaCl aqueous solution (pH=3) were stirred at 1000 rpm to prepare a liquid mixture. The aluminum sheets of Examples 1 and 2 and Comparative Examples 3 and 7 were each immersed in the liquid mixture and removed after a predetermined time period had elapsed. The 10 μL water droplet falling angle of the aluminum sheet of each example was then measured. The coating amount of the automobile engine oil on the aluminum sheet of each example was 3.75 μL/cm².

(Results)

FIG. 3 is a graph showing the relationship between liquid mixture immersion time and 10 μL water droplet falling angle, for each of the aluminum sheets of Examples 1 and 2 and Comparative Examples 5 and 7.

As shown in FIG. 3, within several days the water droplet falling angle exceeded 20° as the measuring limit in all of the examples, but the longest period for maintaining a small water droplet falling angle (that is, maintaining high water-repellency) was exhibited by the aluminum sheet of Example 1 which lacked a SAM coating. On the other hand, the aluminum sheet of Example 2 which had a hierarchical structure and lacked a SAM coating tended to have the greatest increase in water droplet falling (i.e. reduction in water-repellency), exceeding 20° in a shorter time than the aluminum sheet of Comparative Example 7, which was a smooth aluminum sheet lacking both hierarchical structuring and porosity and coated with an automobile engine oil.

It can thus be concluded that when an automobile engine oil is directly coated onto a non-SAM-coated aluminum sheet to form an SLIPS, higher stability in water is obtained without hierarchical structuring.

<Corrosion Resistance Test>

The corrosion resistance of aluminum sheets for Examples 1 and 2 and Comparative Examples 1 to 6 was evaluated in the following manner.

(Test Method)

The aluminum sheet of each example was immersed for 5 days in the aforementioned liquid mixture and the weight change and surface form change of the sheet were observed.

(Results)

The results are shown in Table 1, FIG. 4, FIGS. 5A to 5E and FIGS. 6A to 6E.

TABLE 2 Results Structure Corrosion Hierar- Anod- Engine resistance Example SAM chical ization oil test Comparative Example 1 No No Yes No P Example 1 No No Yes Yes G Comparative Example 2 No Yes Yes No P Example 2 No Yes Yes Yes G Comparative Example 3 Yes No Yes No P Comparative Example 5 Yes No Yes Yes G Comparative Example 4 Yes Yes Yes No P Comparative Example 6 Yes Yes Yes Yes G

In Table 1, “G” represents satisfactory corrosion resistance, and “P” represents no corrosion resistance.

FIG. 4 is a graph showing weight change after a corrosion resistance test for the aluminum sheets of Examples 1 and 2 and Comparative Examples 1 to 6.

As shown in FIG. 4, the weight was reduced by about 0.5 mg/cm² to 1.5 mg/cm² with all of the aluminum sheets of Comparative Example 1, Comparative Example 2, Comparative Example 3 and Comparative Example 4, which were not coated with automobile engine oil. This occurred due to dissolution of aluminum by corrosion.

However, with the aluminum sheets of Example 1 and Example 2, and Comparative Examples 5 and 6, which were coated with automobile engine oil, all of the weight changes were 0.1 mg/cm² or less, and therefore weight reduction by corrosion was inhibited. With the aluminum sheets of Example 2 and Comparative Example 5 and Comparative Example 6 there was a slight increase in weight after the corrosion resistance test. This occurred due to trace white product residue after the corrosion test even after cleaning of the aluminum sheets with an organic solvent such as acetone. The white product is thought to be the reaction product between the solution mixture and the neutralizing agent included among the additives of the automobile engine oil.

These results suggest that coating an automobile engine oil on a pore-formed aluminum sheet can provide corrosion resistance equivalent to SAM coating of the pore-formed aluminum sheet.

The surface states of the aluminum sheets of Example 1, Example 2, Comparative Example 5 and Comparative Example 6 after the corrosion resistance test were observed with a scanning electron microscope.

FIG. 5A is a scanning electron microscope (SEM) image of the surface of an anodized aluminum sheet without hierarchical structuring, before a corrosion resistance test, and FIGS. 5B to 5E are scanning electron microscope (SEM) images of the surfaces of the aluminum sheets of Comparative Example 5, Comparative Example 3, Example 1 and Comparative Example 1, in that order, after the corrosion resistance test.

Comparing the aluminum sheets of Comparative Example 5 and Example 1, which were porous and coated with automobile engine oil on the surface, with reference to FIGS. 5A, 5B and 5D, it can be concluded that with or without SAM coating, a porous layer remained after the corrosion resistance test as before the test, and therefore the pore structure was maintained.

In contrast, as shown in FIG. 5C, the anodized surface dissolved and the pore structure disappeared in Comparative Example 3, which had only SAM coating and did not have automobile engine oil coated on the surface. In Comparative Example 1 which did not have SAM coating and also had no automobile engine oil coated, considerable dissolution occurred not only on the anodized surface but even down to the base aluminum sheet.

FIG. 6A is a scanning electron microscope (SEM) image of the surface of an anodized aluminum sheet with hierarchical structuring, before a corrosion resistance test, and FIGS. 6B to 6E are scanning electron microscope (SEM) images of the surfaces of the aluminum sheets of Comparative Example 6, Comparative Example 4, Example 2 and Comparative Example 2, in that order, after the corrosion resistance test.

When the aluminum sheets of Comparative Example 6 and Example 2, which had hierarchical structures, were porous and were coated with automobile engine oil on the surface, are compared with reference to FIGS. 6A, 6B and 6D, it can be concluded that with or without SAM coating, a porous layer remained after the corrosion resistance test as before the test, and therefore the pore structure was maintained.

In contrast, as shown in FIG. 6C, the anodized surface dissolved and the pore structure disappeared in Comparative Example 4, which had only SAM coating and did not have automobile engine oil coated on the surface. In Comparative Example 2 which did not have SAM coating and also had no automobile engine oil coated, considerable dissolution occurred not only on the anodized surface but even down to the base aluminum sheet.

Based on these results it can be concluded that both with and without a hierarchical structure, coating of an automobile engine oil directly onto the surface of a pore-formed aluminum sheet can provide corrosion resistance equivalent to SAM coating of the surface of a pore-formed aluminum sheet.

Reference Examples 5 and 6

In order to clarify the reason why coating of automobile engine oil directly onto the surface of a pore-formed aluminum sheet provided corrosion resistance equivalent to SAM coating of the surface of a pore-formed aluminum sheet, cleaning with an organic solvent was carried out after coating of the automobile engine oil, and the surface was then analyzed by static contact angle measurement and X-ray photoelectron spectroscopy (XPS).

Reference Example 5

Automobile engine oil was coated onto an aluminum sheet with a porous surface (12.5 μL/cm²) and allowed to stand for 24 hours, as illustrated in FIG. 7. The aluminum sheet was then subjected to ultrasonic cleaning in heptane and dried to obtain an aluminum sheet for Reference Example 5.

Reference Example 6

An aluminum sheet for Reference Example 6 was obtained in the same manner as Reference Example 5, except that it was not coated with automobile engine oil.

<Evaluation of Surface Condition of Aluminum Sheet> (Evaluation Method)

The surface states of the aluminum sheets of Reference Examples 5 and 6 were observed with a scanning electron microscope (SEM). A 10 μL water droplet was placed on each aluminum sheet and the contact angle was measured. The surface of the aluminum sheet of Reference Example 5 after automobile engine oil immersion and cleaning was analyzed by X-ray photoelectron spectroscopy (XPS).

(Results)

FIGS. 8A and 8B are diagrams showing the surface states of the aluminum sheets of Reference Examples 5 and 6, respectively, and the contact angles upon placement of 10 μL water droplets.

As shown in FIG. 8A, since nanopores are clearly observed in the porous surface of Reference Example 5 even after automobile engine oil immersion, it is seen that the automobile engine oil had been removed by cleaning, with no visible changes in pore size.

However, as shown in FIG. 8B, the water droplet contact angle for Reference Example 6 which was not coated with engine oil was 20°, indicating a hydrophilic surface, whereas in Reference Example 5 shown in FIG. 8A, the contact angle was 110° after engine oil immersion and cleaning, showing a clear change in the water-repellent surface.

This suggests that additives in the automobile engine oil were likely chemically adsorbed onto the surface, resulting in surface modification.

FIGS. 9A to 9C are graphs showing X-ray photoelectron spectroscopy (XPS) analysis results for the surface of the aluminum sheet of Reference Example 5, after automobile engine oil immersion and cleaning.

Zn that was not present before automobile engine oil immersion was present as residue after automobile engine oil immersion and cleaning. In the S2p spectrum shown in FIG. 9B, a peak for sulfate ion due to anodization in sulfuric acid is seen near 171 eV, but an additional peak at 164 eV attributed to the additive zinc dialkyldithiophosphate (ZnDTP) is also seen after engine oil immersion and cleaning.

These results suggest that the zinc dialkyldithiophosphate (ZnDTP) in the automobile engine oil was adsorbed onto the surface of the aluminum sheet of Reference Example 5, thus causing surface modification, and that this lowered the surface free energy to produce a stable SLIPS state even without SAM, also leading to significant improvement in corrosion resistance.

Examples 3 to 5 and Comparative Examples 8 to 10

Aluminum sheets for Examples 3 to 5 and Comparative Examples 8 to 10 were fabricated and their corrosion resistance evaluated, in the following manner.

Example 3

An aluminum sheet having a porous anodized surface was fabricated in the same manner as Reference Example 1. The aluminum sheet was coated with oil comprising zinc dialkyldithiophosphate (ZnDTP) added to a base oil, to fabricate an aluminum sheet for Example 3. The amount of zinc dialkyldithiophosphate (ZnDTP) in the oil was 2 mass % with respect to the total oil.

Examples 4 and 5

Aluminum sheets for Examples 4 and 5 were fabricated in the same manner as Example 3, except that the amount of zinc dialkyldithiophosphate (ZnDTP) in the oil was changed to 10 mass % and 20 mass %, respectively.

Comparative Example 8

An aluminum sheet having a porous anodized surface was fabricated in the same manner as Reference Example 1. The aluminum sheet was coated with a base oil to fabricate an aluminum sheet for Comparative Example 8.

Comparative Example 9

An aluminum sheet for Comparative Example 9 was fabricated in the same manner as Example 3, except for using an oil comprising calcium sulfonate instead of zinc dialkyldithiophosphate (ZnDTP) added to a base oil. The amount of calcium sulfonate in the oil was 1 mass % with respect to the total oil.

Comparative Example 10

An aluminum sheet for Comparative Example 10 was fabricated in the same manner as Comparative Example 9, except that the amount of calcium sulfonate in the oil was changed to 10 mass %.

<Evaluation of Corrosion Resistance> (Evaluation Method)

The aluminum sheets of Examples 3 to 5 and Comparative Examples 8 to 10 were evaluated for corrosion resistance by electrochemical measurement. The corrosive solution used was a liquid mixture prepared by stirring acetic acid and a 10 g/L NaCl aqueous solution at 1000 rpm (pH=3).

(Results)

FIG. 10 is a graph showing electrochemical measurement results for the aluminum sheets of Examples 3 to 5 and Comparative Example 8.

As shown in FIG. 10, the aluminum sheet of Comparative Example 8 which was coated with a base oil without addition of zinc dialkyldithiophosphate (ZnDTP) exhibited high anode current density, and a sufficient corrosion-inhibiting effect could not be obtained. In contrast, the aluminum sheets of Examples 3 to 5, which were coated with oils comprising zinc dialkyldithiophosphate (ZnDTP) added to base oils, exhibited current density of at least four digits lower than the aluminum sheet of Comparative Example 8, or in other words, excellent corrosion resistance.

FIG. 11 is a graph showing electrochemical measurement results for the aluminum sheets of Example 3 and Comparative Examples 8 to 10.

As shown in FIG. 11, the aluminum sheet of Comparative Example 8 which was coated with a base oil, and the aluminum sheets of Comparative Examples 9 and 10 which had calcium sulfonate added instead of zinc dialkyldithiophosphate (ZnDTP), exhibited high anode current density, and a sufficient corrosion-inhibiting effect had not been obtained. 

1. A metal member having a porous surface, wherein the porous surface is directly covered by a hydrocarbon-based oil comprising zinc dialkyldithiophosphate (ZnDTP).
 2. The metal member according to claim 1, wherein the porous surface is an oxidized surface.
 3. The metal member according to claim 2, wherein the porous surface is an anodized surface.
 4. The metal member according to claim 1, wherein the metal member is a member of Al, Ti, Fe or Mg, or an alloy of these metals, or stainless steel.
 5. The metal member according to claim 1, wherein the concentration of the zinc dialkyldithiophosphate (ZnDTP) is 0.1 mass % to 30.0 mass % with respect to the hydrocarbon-based oil.
 6. The metal member according to claim 1, wherein the hydrocarbon-based oil is an engine oil.
 7. The metal member according to claim 1, which is a member for an automobile.
 8. The metal member according to claim 7, which is to be used for a part to which an engine oil is to be supplied.
 9. The metal member according to claim 7, which is a member for an intercooler. 